The Next Frontier: The Great Indoors

In 1962, the ecologist Robert Whitaker set out to categorize the different realms of life on Earth. Some were deserts, others tundra, still others tropical forests. He coined a word for these inhabited environments, one that scientists have used ever since: biomes.

The planet’s biomes emerged over hundreds of millions of years. Coastal wetlands sprang up along the edges of continents about 400 million years ago. About 20 million years ago, grasslands became widespread. But the biome that we’re most familiar with — one that has a huge impact on our everyday life — is the youngest of all: the indoor biome.

When humans began building shelters about 20,000 years ago, we unrolled a welcome mat for other species. Over the past few thousand years, the indoor biome has grown to colossal proportions as cities and suburbs spread across the continents. More recently elevators and other technology have lifted the indoor biome into the sky.

If you add up the area of the indoor biome in Manhattan — including its walk-ups and high-rise apartments — it’s three times bigger than the area of the island of Manhattan itself.

We humans are whittling down coastal wetlands, tropical forests and other biomes. But not the indoor biome: Globally, it’s already over 247,000 square miles, bigger than France and growing rapidly. Ours is a biome with a future.

Biologists ignored the Great Indoors in part because it didn’t really seem to be in their job description. Traditionally, they headed into the wilderness to study nature in its pure state.

But recently some scientists have gotten curious about our non-human lodgers, thanks in part to new technology that makes it possible to sample the DNA from any environment. Homeowners, too, are starting to open their doors out of curiosity — even if it means they have to step around the post-docs in the bathroom.

The preliminary results are staggering. One of Ms. Martin’s co-authors, Robert R. Dunn of North Carolina State University, has surveyed houses in North Carolina to catalog the species they contain. He and his colleagues typically find dozens of fungi species in each house, and hundreds of animal species.

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CreditRon Barrett

“They’re much more diverse than anyone had thought,” said Dr. Dunn. Even though he is a trained entomologist, it wasn’t until Dr. Dunn started the research that he realized his own house was home to a species of wasp that lays its eggs inside living cockroaches.

Many of these species may have just ended up with us by accident. Fungal spores may waft in through a window; nematodes may track in on someone’s shoe. But scientists suspect that a number of our co-habitants have adapted to living indoors, faring better inside our buildings than outside.

Dr. Dunn and his colleagues argue that, ecologically speaking, our houses have a lot in common with caves. In both habitats, temperature and humidity are much steadier than outside, making for stable environments. But both lack the dense vegetation that most other biomes have, so there’s less food to be had.

Some species that inhabit caves show striking similarities to those in homes. Many cave animals are flat, as are house-dwelling German cockroaches and bedbugs. In both environments, flatness helps the animals hide in fissures. In both places, many animals have poor eyesight, relying instead on antennae and other sense organs.

But our houses also have otherworldly ecological niches, like shower heads and freezers, that can support more biological diversity than you’d find in a cave.

It’s possible that some species already were adapted for living indoors when they first turned up on our doorsteps. Grain beetles, for example, are notorious pests in farm buildings and kitchen pantries. Before becoming homebodies, they raided rat nests and ant colonies to steal their stored seeds — not all that different from what they do now.

Likewise, a black yeast called Exophiala may have arrived in houses already equipped with many traits it needed to thrive in sinks and dishwashers. In nature, Exophiala grows on the skin of tropical fruit, and so it developed a talent for sticking to surfaces. In part of its life cycle, the yeast passes through the guts of fruit-eating animals, so it already withstood heat and high acidity.

That doesn’t mean that evolution stops as soon as a species moves indoors. In recent years, German cockroaches have evolved a distaste for sugar. That’s because exterminators started using sugary bait in roach traps.

It’s a mystery just how much adaptation happened after life colonized the indoor biome and how much was already there, because scientists understand so little about indoor species. Even the familiar German cockroach is an enigma.

It belongs to a genus called Blattella, as do 50 other species. Nobody knows which one is the closest relative to German cockroaches, so it’s hard to say how much this pest has been shaped by cohabitation with humans.

This research may eventually help us engineer the indoor biome to push pathogens out of our homes. “We will have an ability to start to design healthier buildings,” Jack Gilbert, a microbiologist at Argonne National Laboratory.

But before we can tend these indoor gardens, we need to know what’s growing in them.